The broad objectives of the proposed investigations are to characterize the nature of antibody and T cell mediated immune responses to infection with respiratory syncytial virus (RSV) in infancy and childhood. It is also planned to examine the effects of pre-existing RSV specific immunologic reactivity on the outcome of subsequent RSV infection in man and in experimental settings in an animal model. The specific studies proposed in the original proposal included the following. 1. Developmental characteristics of cell mediated and antibody response in infants infected with RSV. It was proposed to examine the RSV specific immune response in relation to a) degree of viral replication and antigen load, b) alterations of RSV specific immunologic reactivity relative to the nature of clinical disease, and c) nature and the role of RSV specific secretory immune response. 2. Natural history of RSV infection-immune response interaction in infancy and early childhood. Specific longitudinal studies have been designed to delineate the mechanism responsible for development of severe clinical disease with RSV in early infancy. These studies will explore a) nature of immunologic reactivity in primary vs. secondary infection with RSV, b) clinical outcome of primary vs. secondary infection with RSV, c) effects of pre-existing serum and secretory immunologic reactivity on the outcome of RSV infection in the infant.